Heat transfer media



N02 ZING POINT 2 1C.

INVENTOR. Ffa/ms H. .IA/SINGER, Je. BY i Z E l Oct. 19, 1954 H |N51NGER,JR

HEAT TRANSFER MEDIA Filed NOV. 5, 1949 LNO5 J m @l c. n SVMM in A AAA wom a T YA. A f, l m VAV f a m AVA AV A G una An. A y m YAY. A m AVWVAV MAN W o F .fw YAY NA ,Vv u vw w n Awww Patented Oct. 19, 1954 HEAT TRANSFER MEDIA Thomas Harold Insinger, Jr., Sewickley, Pa., as-

signor to Koppers Company, Inc., Pittsburgh, Pa., a corporation of Delaware Application November 5, 1949, Serial No. 125,802

2 Claims. 1

This invention relates to heat transfer media. More particularly the invention relates to high temperature heat transfer meda which have solidication points below 140 C.

Many chemical reactions may be carried out more advantageously when the reaction products are heated to uniformly controlled temperatures. Some inorganic salts in molten state have been used as heat transfer media, but many of the molten salt solutions heretofore used for operating at temperatures between 400 and 500 C. have high solidication points which make them inconvenient to handle.

To permit the molten inorganic salt solutions to be safely used as heat transfer media, it is customary to circulate the media in a tightly closed circuit immersed in or enclosing the material to be heated and to circulate the molten salt solution in a closed cycle between a heat source and the reaction vessel, so that the heat is transferred through a diaphragm. If it becomes necessary to discontinue circulation of the molten salt solution and it is allowed to cool it will solidify. Since the molten salt solution generally fills most or all of the coil or circuit container, it is very difficult to again melt a high solidication temperature transfer media so that it may be again put into circulation.

The primary object of the present invention is to provide high temperature heat transfer media that have solidiiication points below 100 C. or the temperature of saturated steam at atmospheric pressure.

Another object of the invention is to provide high temperature heat transfer media that have solidication points below the temperatureA of moderate pressure dry steam, that is, the temperature of dry saturated steam at a gauge pressure of 50 pounds per square inch.

Another object of the invention is to provide a heating medium which is thermally stable and capable of transferring heat at a temperature above 450 C., and which has a relatively low melting point.

With these and other objects in view, the invention consists in the high temperature heat transfer media hereinafter described and particularly defined in the claims.

The heat transfer media of the present invention are ternary mixtures of lithium nitrate, potassium nitrate and sodium nitrite. These three inorganic salts form a ternary eutectic which does not solidify at temperatures above 70 C. and the media formed by different mixtures are capable of stable operation at temperatures above 450 C.

Composition Solidiiication Point, C.

LiNOa,

NaN Oz, Percent Percent KNOa, Percent In the accompanying drawing the solidication points shown in the above table have been plotted on a chart of triangular coordinate paper which indicates the composition of each mixture (eX- pressed as percent by weight) corresponding to the solidification point of the mixture. A triangular plot is shown on the chart in dot-anddash lines with one apex being a mixture composed of LiNOa 55%, NaNOz 20%, and KNOs 25%. Another apex is a mixture having a solidication temperature of 78 C. composed of LiNOs 10%, NaNOz 20% and KNOa r70%. A third apex is a mixture composed of LiNOs 10%, NaNOz 65% and KNOS 25%.Y All of the mixtures within the triangular area have solidication temperatures below 100 C. It is evident, however, that there could exist some mixtures having solidiiication temperatures of 100 C. or lower that would be outside, but close to the boundaries of the triangular plot. The majority of the mixtures which have solidication temperatures between and 100 C. are made up of LiNOa 10% to 3 55%, NaNO2 20% to 65%, and KNO3 25% to '70%. All of these mixtures which have solidication points below 100 C. are capable of operating as heating media at temperatures above 450 C.

It will be observed that compositions falling within the above described triangular plot but having not more than lithium nitrate have the advantage of having the desired low solidiication temperatures of between '70 and 100 C. and the further advantage that this result is obtained With less lithium nitrate, which is by far the most expensive ingredient, than needed to form the binary eutectic of lithium nitrate land potassium nitrate which has the lowest solidification point known in the art for any of the three components or mixtures thereof. Thus by replacing part of the relatively expensive ingredient, lithium nitrate, by sodium nitrite or sodium nitrite and potassiumv nitrate, compositions according to this invention are obtained which are substantially .more economical and .at the same time have markedly lower solidification points.

Below the triangular plot on the chart is a trapezoidal area of mixtures having solidication Ypoints from about 80 C. up to 140 C. The two upper corners of the trapezoid are first, a mixture composed of LiNO3 10%, NaNO2 25%, and

'KNO3 65%, and second a mixture having a solidi- "icationpoint of 139 C. composed of LiNOs 10%s NaNO2 70% and KNOs 20%. A third and lower :corner is a mixture of LiNOs 2%, NaNOz r10% Vand KNOa 28%- ,is:a mixture compound of LiNOa 2%, NaNO2 25% `and KNO3'73 yThe mixtures within the trape- .zoidal area which have been examined all have The fourth and lower corner solidication temperatures below 140 C. and are capable .of operating as heating media at temperatures of 450 to 500 C.

Those mixtures .of lithium nitrate, sodium nitrite and potassium nitrate which come within the area bounded by the dot-and-dash triangle lare suitable for .use as heat, transfer media in an equipment which can be heated withsaturated steam at atmospheric pressure or thereabouts to 'bring the heat transfer meda into a liquid state for circulation ina closed heating cycle. The lmixtures of lithium nitrate, potassium nitrate and sodium nitrite included in the trapezoidal area shown in the chart areipreferablyemployed vin apparatus which can be heated with dry saturated steam at a gauge'prerssure of 50 lbs. per square inch to prepare the heating media in condition for circulation A`in a closed heating cycle. -The eutectic temperature for a mixture of potassium nitrate and lithium nitrate has been found to be 129 C. The eutectic temperature for a mixture of potassium nitrate and sodium nitrite has been found to be 139 C. It is believed that the eutectic temperature of a mixture of lithium nitrate and sodium nitrite has not been accurately determined up to the present time. `Lithium nitrate startsto decompose at about 475 C. Potassium nitrate 'has the decomposition temperatul'e of 532.7 C. Sodium nitrite does notdecompose to yield NO2 at temperatures below 600 C.

Any mixture of the components lithium nitrate, potassium nitrate and sodium nitrite as shown the-table' and -on the plot is capable of substantially stableoperation at temperatures of 450 C. for Vanextended period of time. It will be understood however, that for high temperature operations the minimum amount of lithium nitrate is used on account of the cost of this component. v

".The preferred form of .the .invention having been thus described, `what is claimed Vasinevv is:

1. A ternary mixture of approximately 16% by weight of klithium nitrate, 42%;by. weight; of potassium nitrate,.and 42% by Weight of'fsodium nitrite.

2. A low fsolidication point fusible vcomposition consisting of Va .ternary .mixture-of .lithium nitrate, sodium nitrite and potassium nitrate having a Ipercentage composition falling within the area on' the chart of the .drawing bounded substantially by the triangle :formed by lines connecting the following points:

yLiN'oa, .'NaNo., KNol lercent Percent I -Percent i 25 l44.8 30.2 15 55 30 1e 38.6 v .45.4

References cited in the .fue of thispatent UNITED STATES PATENTS Number 'Name Y Date -1,555,400 Burns Sept.V 29, 1925 1,984,369 Gensch Dec. 18," 1934 2,148,664 Wille. et al Feb. 28, 1939 2,205,180 Shaver June 18,Y 1940 '2,375,759 Kalichevsky-et al. May 15, 1945 2,429,129 Heppell et al. Oct. 14, 1947 FOREIGN PATENTS Number Country .'Date 110,773 Australia June2, 1940 214,894 Switzerland Aug.f16, 1941 

1. A TERNARY MIXTURE OF APPROXIMATELY 16% BY WEIGHT OF LITHIUM NITRATE, 42% BY WEIGHT OF POTASSIUM NITRATE, AND 42% BY WEIGHT OF SODIUM NITRITE. 